Moineau pump-type progressive cavity displacement motors have been used in oil and gas well drilling operations for some time. In these downhole drilling operations, drilling rig pumps pump drilling fluids, such as drilling mud, downwards through drill pipe to progressive cavity motors located downhole near the end of the drill string. Commonly, a progressive cavity displacement motor is part of a drilling assembly and serves as a drilling or mud motor that drives a drill bit which bores a hole through the underground formation. The pumped drilling fluid powers the mud motor by spinning a rotor within a stator assembly. The rotor and stator constitute the power section of the mud motor.
Typically, progressive cavity displacement motors are configured with helical metal lobed rotors that turn within elastomeric stators. Stators typically consist of rubber with high carbon black filler content. The high carbon black rubber provides a suitable yet cost efficient material having some compressive modulus and abrasion resistance properties. As the metal lobes of the rotors press against the elastomeric stator inner walls, a sealing line is formed and fluid is thus pumped through the cavities as they are formed between the metal lobes of the rotors and the elastomeric stator inner walls.
Usually, the stator is manufactured by attaching a mold to the inner bore of the stator tube and injection molding an uncured elastomer compound into the mold cavity. A challenge to producing a high power, high torque, and high speed power section stator, is that manufacturing equipment and cost effective tooling materials require a low viscosity uncured elastomer compound that is capable of flowing through a tight mold cavity over a long distance while maintaining its uncured state. If the compound is too viscous it cannot flow the appropriate distance along the length of the stator to fill the mold. If a compound begins the vulcanization reaction before the mold is filled, the compound will increase in viscosity, possibly resulting with a mold that is not filled, or a mold that will fill with cross-links that cluster in separate matrices. Separately formed matrices create undetectable grain boundaries in the elastomer product which will often fail prematurely due to significant losses in tear resistance, losses in modulus, and or friction points internally that facilitate rapid physical deterioration of the surrounding elastomer matrix. Traditionally, designers of power section elastomers have sought to address these issues using reinforcing and semi-reinforcing carbon blacks, low viscosity low molecular weight base NBR and HNBR polymers, and process aids in the recipe. Although such combinations are favorable for manufacturability, the resulting recipe negatively impacts the final cured state properties of the elastomer, often making the formulation softer and less dynamically stable. For example, plasticizer oil may be used to decrease viscosity during manufacturing but, in the finished product, it has a tendency to leach out of the elastomer at high temperatures when exposed to diverse drilling fluids, which can cause shrinkage in the product or de-bonding of the rubber to metal bonding agents, and also facilitate the absorption of chemicals from drilling fluid. Plasticizers are used to reduce the viscosity of an uncured rubber compound by lubricating between the polymer chains and aiding in the dispersion of carbon blacks. Once in cured state, plasticizers continue to lubricate the polymer chains creating an effect of lowered modulus. Additionally, plasticizers, being significantly lower molecular weight than polymers, can migrate out of a compound. Controlling the migration of plasticizers is a function of choosing a plasticizer with the right molecular mass/branching and carbon-to-oxygen ratio for a particular compound. The more branching a plasticizer has, the more resistant the plasticizer is to fluid extraction in oils. The potential to react an ester-based plasticizer into the polymer matrix will substantially increase the resistance to extraction.
Elastomeric compounds have seen the incorporation of phenolic resins which reduces the uncured viscosity of the compound and increases the hardness of the cured state product. But this is generally at the cost of reduced tear resistance. Elastomeric compounds have also seen some use of nanoparticles; however, due to the extraordinary surface area to particle volume (i.e. aspect ratio), these compounds can greatly increase the viscosity of the elastomer with only small amounts of additive nanoparticles. This means their potential in power sections stator compounds requires such low loadings (to maintain manufacturability) that the cured state physical properties are not attainable at an affordable, reproducible level of satisfaction.
Further, though the helical metal rotors of progressive cavity motors are heat tolerant, abrasion resistant, and have generally long useful lives, the stators of progressive cavity motors are far less reliable and often fail, need servicing, or replacement before their rotor counterparts. The carbon black reinforced liner of stators tends to wear down when exposed to abrasive materials, can develop leaks between cavities. When exposed to harsh temperatures a rubber compound will soften and can result in seal lines being less capable of handling high differential pressure which can result in a loss in torque. High temperatures can also cause the rubber/elastomer in a liner to thermally expand and thermally soften, which can lead to overheating. Long term exposure to such conditions can cause the rubber to become brittle and lead to low tear resistance. Failures can occur in the form of a section worn down by abrasion leaking and not providing proper sealing pressure against the metal rotor lobes; a physical tear of the inner lining can also occur and cause an immediate shutdown of the entire system. For example, when the stator fails, the rotor can pump torn rubber pieces through cavities and damage other components of the downhole assembly or stop rotating all together. Exposure to certain chemicals or downhole fluids can additionally cause degradation of the stator inner walls. Harsh drilling fluids can be absorbed into the rubber liner, causing swelling which leads to the rubber liner overheating in operation. Fluids can also extract chemicals from the rubber, thereby degrading it.
The power output, efficiency and torque of progressive cavity positive displacement motors is related to the cross sectional area of the stator and rotor that is available for fluid flow, as well as the ability of the rotor and stator to seal against one another and prevent the pressurized fluid from leaking out into low pressure areas of the motor. Because of dimensional limitations of the wellbore, and the structural and functional requirements of the stator and rotor, the flow cross sectional area can be limited. Also, strength limitations and local failures in elastomer integrity can allow drilling fluid leaks at moderate pressure differentials. Accordingly, such a motor may be limited to generating only moderate torque output. If the torque that the motor must overcome exceeds the torque the motor can produce, the motor may stall, rupturing the power section seals and causing severe damage to the power section stator.
It would thus be desirable to have a more robust progressive cavity positive displacement motor with increased power output, efficiency and torque output, as well as improved heat tolerance, abrasion resistance, tear resistance, and other beneficial properties. Further, it would be desirable to provide increased meantime between failures, increased reliability, and an expectation of extended runtime for operations running elastomeric stator assemblies downhole. This would allow greater drilling time and decreased time spent installing, retrieving, and servicing elastomeric stator assemblies and other components of the associated downhole assemblies that can fail as the result of a stator failure. It would further be desirable to increase the predicted time interval between required servicing of elastomeric stator assemblies.